A semiconductor mechanical quantity sensor has been disclosed in, for example, JP-A-5-304303. A mono-axial (X-direction) capacitor-type acceleration sensor will be generally described with reference to FIGS. 3A–3C. Grooves 11 are formed in a semiconductor layer of a semiconductor substrate 10 such as of silicon, whereby a plurality of pairs of fixed electrodes 1 and moving electrodes 2 are opposed to each other in the X-direction to constitute capacitors. The moving electrodes 2 are formed in a plurality of pairs like a comb teeth in the ±Y-direction with respect to a weight 3 extending in the X-direction. Both ends of the weight 3 are formed on the semiconductor substrate 10 so as to undergo a displacement in the X-direction, and beams 4 of a two-piece structure are formed at both ends of the weight 3 so as to undergo the displacement depending upon the acceleration. The fixed electrodes 1 arranged in the ±Y-direction so as to be opposed to the moving electrodes are connected to pads 5a and 5b made of aluminum or the like, and the moving electrodes 2 are connected to a pad 5c. The pads 5a, 5b and 5c are connected to an external unit through pads 6a, 6b and 6c of another circuit chip 6 such as a mother board by bonding using wires W.
Here, a moving electrode 2a is arranged between the neighboring fixed electrodes 1a and 1b. When an acceleration in the X-direction is exerted on the sensor of this constitution, the beams 4 are displaced in the X-direction, whereby distances vary among the fixed electrodes 1a, 1b and the moving electrode 2a, causing a change in the capacitance CS1 between the fixed electrode 1a and the moving electrode 2a and in the capacitance CS2 between the fixed electrode 1b and the moving electrode 2a. An equivalent circuit of the semiconductor mechanical quantity sensor is illustrated on the left side in FIG. 4. A pulse voltage Vcc has been applied across the fixed electrodes 1a and 1b. A change ΔC (=CS1-CS2) in the capacitances CS1 and CS2 that has occurred is taken out from the moving electrode 2, and is converted into a voltage=(CS1−S2)·Vcc/Cf through, for example, a switched capacitor circuit 5 illustrated on the right side in FIG. 4 to thereby detect the acceleration.
In order to improve the sensitivity of the sensor, so far, it was attempted to soften the spring constant kw by varying the sizes of beams 4, electrodes 1, 2, and weight 3 of the comb teeth structure, by increasing the mass m or by increasing the capacitance C0. FIGS. 5A–5C illustrate a structure in which the beams 4 are folded twice to soften the spring constant of the beams 4 to be one-half in an attempt to double the sensitivity.
However, the resilient restoring force <electrostatic force between the fixed electrodes 1 and the moving electrodes 2 involves a problem of easy sticking. Further, the circuit chip, too, easily undergoes the displacement in the vertical direction (Z-direction). When a large shock is exerted in the Z-direction, therefore, the moving electrodes 2 ride on the fixed electrodes 1 and become no longer capable of moving. Also, the dynamic range narrows.